Process and device for production of lng by removal of freezable solids

ABSTRACT

Novel processes and devices for the removal of freezable species such as carbon dioxide, water and heavy hydrocarbons from a natural gas feed stream during liquefaction to produce LNG are disclosed. The freezable species are able to be removed as a solid, avoiding the costly step of pretreatment to remove the freezable species from the natural gas feed stream prior to the liquefaction stage. The freezable species may be removed on a continuous basis being separated as solids following liquefaction of the natural gas feed stream with subsequent separation of the solids. The solid freezable species may then be liquefied on a continuous basis if required with natural gas recycled to the process. Continuous removal of the freezable species from the natural gas feed stream is achievable by maintaining cooling and separation apparatus at the same working pressure. Advantageously, at least part of the cooling vessel is constructed from a material having a low thermal conductivity which discourages formation of the solids of the freezable species on the walls of the cooling vessel.

FIELD OF THE INVENTION

The present invention relates to a process and device for the removal offreezable species such as carbon dioxide, water and heavy hydrocarbonsfrom a natural gas feed stream and more particularly to a process forthe removal of freezable species from the natural gas feed during theliquefaction of the natural gas to produce LNG.

BACKGROUND OF THE INVENTION

In conventional LNG plants, heat transfer for cooling a natural gas feedstream sufficiently to form a liquid is effected in a heat exchanger.Natural gas can contain a wide range of compositions of species whichare capable of forming solids during the cryogenic process of liquefyingnatural gas. Such species are referred to throughout this specificationas “freezable species” and the solids formed of the freezable speciesare referred to as “freezable solids”.

Freezable species which are not removed prior to entering the cryogenicLNG cooling vessel precipitate and accumulate on the cold surfaces ofthe heat exchangers and other equipment, eventually rendering theseitems inoperable. When fouling has reached a sufficient level, thecooling vessel must be taken off-line for the fouling to be removed. Inthe process the cooling vessel, baffles or pipework can be damaged whichonly encourages further fouling in the next production cycle. Moreover,solids condensing on metal surfaces form an insulating film reducingthermal efficiency of the heat exchanger.

In a conventional LNG facility, pre-treatment of the natural gas isrequired to remove the freezable species prior to the natural gas feedstream being directed to the cooling stages to cause liquefaction. In atypical natural gas, the CO₂ composition can range between 0.5% to 30%and can be as high as 70% in commercially viable reservoirs like Natuna.In a conventional LNG facility, the level of CO₂ present in the naturalgas is typically reduced down to the level of 50 to 125 ppm prior to thenatural gas feed stream being directed to liquefaction. Another of thefreezable species, namely hydrogen sulphide (H₂S), is normally removeddown to a level of 3.5 mg/Nm³ prior to the natural gas feed stream beingallowed to enter the liquefaction stage. One of the methods typicallyused to remove the freezable species from the natural gas feed stream isa chemical reaction using reversible absorption processes such asabsorption with an amine solvent.

This is an expensive and complex process and commonly encountersoperational problems such as foaming, corrosion, blocked filters, aminedegradation, and losses of amine, water and hydrocarbons. The processalso consumes energy to regenerate and pump the solvent Treated gas fromthe amine system will be water saturated and needs to be dried to lessthan 1 ppm prior to liquefaction. This is normally achieved by usingfixed-bed solid adsorbents such as molecular sieves.

The natural gas feed stream is sometimes pre-treated to partially removewater along with some heavy hydrocarbons by means of a pre-cooling cyclefrom the main refrigeration unit. Alternatively, Joule-Thomson coolingcan be used if excess feed gas pressure is available. Care must howeverbe taken to keep the gas above the hydrate formation temperature. Thisis again a relatively expensive process. Large insulated pressurevessels are required along with a regeneration system. Regeneration ofthe molecular sieve is required and this consumes energy to heat thegas. The regenerated gas must be heated prior to entering the “wet”adsorption unit, then cooled to remove water before it is recycled(usually compressed) to the inlet of the duty adsorption unit. If amolecular sieve is used to remove CO₂, the regeneration gas must bedisposed of or used as fuel gas.

Heavy hydrocarbons (typically C₆+) are typically partially removed alongwith water as explained above. Where further removal is required, acryogenic distillation column is required, with cooling provided fromthe main refrigerant cycle. Again, this can be an expensive and complexprocess, especially if the removed components are required forrefrigerant make-up in a mixed refrigerant cycle.

An attempt has been made to develop a process for removing the freezablespecies during the liquefaction stage as described in WO 99/01706 (Coleet al. The distallative separation process of Cole et al includes acontrolled freezing zone in which the freezable species both solidifyand subsequently melt prior to distallative separation in the bottomhalf of the column. The freezable species are removed in the form of aliquid via a bottoms product enriched in the freezable species.

There are no known techniques for removing the freezable species duringliquefaction with the freezable species remaining in solid form.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of removal of the freezable species from the natural gas feedstream, the method comprising the steps of:

-   -   cooling the feed stream in a cooling vessel to produce        pressurised LNG in a manner such that the freezable species        solidify forming a slurry with the pressurised LNG; and,    -   removing the slurry from the cooling vessel whilst maintaining        the freezable species as a solid.

The step of cooling the feed stream to produce pressurised LNG isreferred to in the specification as “liquefaction”. The temperature andpressure at which liquefaction is conducted is not a critical parameterof the present invention, but by definition, any of the species capableof forming solids during the liquefaction of a natural gas to form LNGwill form solids. The freezable species may include but are not limitedto CO₂ and H₂S.

Preferably, the step of cooling is conducted in a manner such that thefreezable species solidify away from a wall of the cooling vessel.Throughout this specification the zone in which the freezable speciessolidify within the cooling vessel is referred to as the “solidificationzone”. One way of promoting the formation of the solidification of thefreezable species away from the walls of the cooling vessels is tomaintain a temperature gradient within the cooling vessel such that thetemperature towards the centre of the cooling vessel is less than thetemperature at the wall of the cooling vessel. In practice, one way ofachieving this is to use a material of construction for at least theinternal wall of the cooling vessel surrounding the solidification zonefrom a material having a low thermal conductivity.

Preferably, the process further comprises the step of separating thesolids of the freezable species from the slurry. More preferably, thestep of removing the slurry from the cooling vessel is conductedsimultaneously with the step of separating the freezable species fromthe slurry.

According to a second aspect of the present invention, there is provideda process for the continuous removal of a freezable species from anatural gas feed stream comprising the steps of:

-   -   cooling the feed stream in a cooling vessel to produce        pressurised LNG in a manner such that the freezable species        solidify forming a slurry with the pressurised LNG; and,    -   separating the solids of the freezable species from the slurry,        wherein the step of cooling and the step of separating are        conducted at the same working pressure.

When the steps of cooling-separating are conducted at the same workingpressure, ie the pressure in use, it is possible to run the process in acontinuous manner as opposed to a batch operation. The word “continuous”does not imply that the process would at no time be halted. In anyprocess, it will be necessary to stop production for various reasons,for example maintenance.

Preferably, the process for continuous removal of the freezable speciesfurther comprises the step of heating the separated solids of thefreezable species to form a liquid of the freezable species and thesteps of cooling, separating and heating are conducted at the sameworking pressure.

It is to be understood that the “same working pressure” is used todescribe the situation whereby the pressure in use is adjustablymaintained such that all three stages are at all times at equilibrium.The working pressure itself may vary.

It is highly preferable that the pressure is maintained at all timesbelow the triple-point pressure of the freezable species. This is doneto ensure that a vapour phase of the freezable species is not formed,which would require a further separation step before the natural gasvapour formed during the step of heating the solids of the freezablespecies could be recycled to the process.

Preferably, the process as defined in either one of the first or secondaspects of the present invention further comprises the step of recyclingto the cooling vessel LNG from which the freezable species has beenseparated. Preferably, the process also comprises the step of recyclingto the cooling vessel natural gas from which the freezable species hasbeen separated during the step of heating of the slurry to liquefy thefreezable species.

Preferably, the process defined in either of the aspects described abovefurther comprises the step of creating a vortex within the coolingvessel. Preferably, the vortex is created by stirring the slurry.Alternatively or additionally, the vortex may be created by introducinga fluid stream tangentially to the cooling vessel. Preferably, the fluidstream introduced tangentially to the cooling vessel is a stream ofsub-cooled LNG. The stream of sub-cooled LNG may be the sub-cooled LNGstream recycled after separation of the freezable species from theslurry.

Preferably, the step of cooling comprises the step of isotropicallyexpanding the feed stream.

Preferably, the step of cooling alternatively or additionally comprisesthe step of introducing a stream of sub-cooled LNG. Advantageously, thestream of sub-cooled LNG introduced to assist in the step of cooling maybe the stream of recycled LNG separated from the slurry during the stepof separating the solids of the freezable species.

According to a third aspect of the present invention, there is providedan apparatus for removing a freezable species from a natural gas feedstream, the apparatus comprising:

-   -   a cooling vessel having a solidification zone therewithin        wherein that part of the cooling vessel that surrounds the        solidification zone is constructed from a material having a low        thermal conductivity;    -   an inlet for introducing the feed stream to the cooling vessel;        and,    -   an outlet for removing a slurry of solidified freezable species        and pressurised LNG from the cooling vessel.

The solidification zone is defined above as that part of the coolingvessel within which the freezable species forms solids within thecooling vessel during cooling. The extent of the solidification zonewill depend on the size of the cooling vessel, the temperature andpressure of operation of the cooling vessel and the particular freezablespecies present within the particular natural gas feed stream.

It is to be understood that while the entire cooling vessel may beconstructed of a material having a low thermal conductivity, it is onlynecessary for the working of the present invention for that part of thecooling vessel that surrounds the solidification zone to be constructedfrom such a material. Moreover, it is the surface within the coolingvessel, ie the internal wall of the cooling vessel, thus must beconstructed of the material having a low thermal conductivity to achievethe claimed result. Thus it is to be understood that the cooling vesselcould be constructed of a material having a high thermal conductivity asthe outer case of such a cooling vessel, provided that the internal partof the cooling vessel that surrounds the solidification zone isconstructed with the material having a low thermal conductivity.

One of the advantages of constructing the material in such a way is thata thermal gradient is developed within the cooling vessel whereby thetemperature towards the centre of the cooling vessel is at all timescooler than the temperature at the walls of the cooling vessel. Theresult of this is that the freezable solids form preferentially towardsthe centre of the cooling vessel and away from the walls of the coolingvessel, reducing or eliminating fouling of the cooling vessel due tosolidification of the freezable species on the plant equipment itself.

Preferably, the apparatus further comprises a solid/liquid separator forseparating the solidified freezable species from the slurry. Morepreferably, the separator is located at and/or defines the outlet. Theseparator may be one of a plurality of separators arranged in series orin parallel.

Preferably an expansion valve is located at and/or defines the inlet forintroducing the feed stream to the cooling vessel. One suitableexpansion valve is a Joule-Thompson valve for isotropically expandingthe natural gas feed stream upon entering the cooling vessel.Introducing the natural gas in this way results in cooling of thenatural gas feed stream.

Preferably, the apparatus further comprises a stirrer for creating avortex within the cooling vessel in use. Alternatively oradvantageously, the cooling vessel may further comprise a tangentialinlet for introducing a fluid into the cooling vessel for creating avortex within the cooling vessel in use. Preferably, the fluid streamintroduced into the cooling vessel will be a stream of sub-cooled LNG.The sub-cooled LNG stream may be recycled from other stages of theprocess.

According to a fourth aspect of the present invention, there is providedan apparatus for continuously removing a freezable species from anatural gas feed stream the apparatus comprising:

-   -   a cooling vessel having a solidification zone therewithin        wherein that part of the cooling vessel that surrounds the        solidification zone is constructed from a material having a low        thermal conductivity;    -   an inlet for introducing the feed stream to the cooling vessel;    -   an outlet for removing a slurry of solidified freezable species        and pressurised LNG from the cooling vessel; and    -   a solids collection vessel in fluid communication with the        cooling vessel.

Maintaining the cooling vessel and the solids collection vessel in fluidor hydraulic communication will have the result that each of thesevessels operates at the same working pressure.

Preferably, the apparatus further comprises a transfer means fortransferring the slurry from the cooling vessel to the solids collectionvessel.

Preferably, the transfer means is inclined at an angle. As the slurrytravels via the inclined transfer means from the cooling vessel to thesolids collection vessel, pressurised LNG is removed from the slurryunder gravity increasing the concentration of the solids in the slurryto produce a slurry that is highly concentrated in solids, hereinafterreferred to throughout this specification as a “slush”. More preferably,the transfer means is inclined at an angle not less than 60° to thehorizontal reference plane. Preferably, the transfer means is providedwith an external drive.

Preferably, the material of construction of an internal wall of thecooling vessel of the third or fourth aspect of the present invention ispolished and, more preferably, highly polished.

Preferably, the material of construction of the internal wall of thecooling vessel of the third or fourth aspect of the present inventionhaving a low thermal conductivity is anisotropic. The material ofconstruction may be a metal oxide or a ceramic. More preferably, thematerial of construction is a single crystal. One suitable material ofconstruction is sapphire.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now the described,by way of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram of an apparatus for removing a freezablespecies from a natural gas feed stream in accordance with a firstpreferred embodiment of the present invention.

FIG. 2 is a schematic diagram of a cooling vessel including a stirrerfor creating a vortex and a sub-cooled LNG stream in accordance with asecond preferred embodiment

FIG. 3 is a schematic diagram representing a process of LNG liquefactionin accordance with a third embodiment of the present invention includingan integral cyclone and a tangential inlet for introducing a sub-cooledLNG stream.

FIG. 4 is a schematic diagram of an apparatus in accordance with afourth preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

An apparatus 10 for removing a freezable species from a natural gas feedstream 12 is depicted in FIG. 1. The apparatus 10 includes a coolingvessel 11 in which the feed stream 12 is cooled to produce pressurisedLNG.

Prior to its introduction to the cooling vessel 11, the natural gas feedstream 12 will typically be dried to produce a water content of lessthan 50 ppm. Any suitable process for drying the natural gas feed streammay be employed. One method of removing water from the natural gas feedstream is to use fixed-bed solid absorbents or other dehydrationprocesses such as dehydration using glycol or methanol.

Another method of removing the water is to capture the water ingas/hydrate form. This method of removing water comprises cooling thenatural gas by passing it over a cold surface at a temperature of −15°C. sufficient to freeze the water molecules adjacent to the gas contactsurface so that ice in the form of hydrate is deposited on the gascontact surface along the gas flow path.

Whilst the natural gas feed stream would typically be dehydrated toremove water, this is not considered an essential step of the presentinvention and the natural gas feed stream 12 entering the cooling vessel11 may contain water as one of the freezable species. The bulk of thisdiscussion, however, will be centred on the removal of CO₂ as thefreezable species. However, it is to be understood that the freezablespecies may include hydrogen sulphide, mercury and heavy hydrocarbons.

The temperature and pressure conditions of the natural gas feed stream12 prior to the entry of the feed stream into the cooling vessel 11 mustbe such that the CO₂ and other freezable species are not allowed to formsolids upstream of the cooling vessel 11. This is done by ensuring thatthe equipment upstream of the cooling vessel 11 is operated at atemperature typically in excess of −52° C.

By definition, under conditions conducive to form pressurised LNG withinthe cooling vessel 11, the freezable species present within the naturalgas feed stream 12 will solidify. The region within the cooling vessel11 in which the freezable solids solidify is referred to as the“solidification zone” 13. As depicted in each of FIGS. 1 to 4, thesolidification zone 13 within the cooling vessel 10 is effectively anopen space within the cooling vessel with no trays or plates or otherphysical barriers of any kind.

The material of construction of at least that part of the internal wallof the cooling vessel 11 in the area of the solidification zone 13 willbe of any material able to withstand the process conditions of pressureand temperature required to perform liquefaction of the natural gasprovided that the material has a sufficiently low thermal conductivitythat the temperature is at all times cooler towards the centre of thecooling vessel than the temperature at the wall of the cooling vessel insolidification zone 13.

The freezable species will then preferentially form solids away from thewall of the cooling vessel 11 surrounding the solidification zone 13 dueat least in part to the thermal gradient being maintained such that thetemperature towards the centre of the cooling vessel is at all timesless than the temperature at the walls.

In accordance with the first preferred embodiment of the presentinvention as illustrated in FIG. 1, the natural gas feed stream 12 isexpanded into the cooling vessel 11 through a Joule-Thompson valve 20.The natural gas feed stream 12 is maintained at a constant pressureimmediately upstream of the Joule-Thompson valve 20 to ensure controlledexpansion of the natural gas from the pressure upstream of the coolingvessel at inlet pipe 22 to the lower pressure within the cooling vessel11 following expansion through the valve 20.

Tests conducted by the applicant have shown that the optimum results forliquefaction are: obtained using an inlet gas pressure to the expansionvalve 20 of between 200 and 600 psia. At these operating pressures, thetemperature of the natural gas feed stream 12 upstream of the expansionvalve 20 must not be allowed to fall below the critical temperature of−56° C. at which CO₂ begins to freeze.

In the case of the freezable species being hydrogen sulphide, thefreezing point for pure H₂S at 14.5 psia is 82.9° C. Hydrogen sulphidehas a vapour pressure of 271 psia at 20° C. It is thus understood thatif H₂S is present in the initial natural gas feed stream 12, solids ofH₂S will form solids in the cooling vessel 11 during liquefaction of thenatural gas feed stream 12. In the case of mercury, even very lowamounts of mercury in the natural gas feed stream is known to causecorrosion of the traditional material of construction of coolingvessels, namely aluminium alloys. Mercury-induced corrosion,particularly in the presence of water, has been known for some time, butthe specific corrosion mechanism is not fully understood. Mercuryremoval from the natural gas feed streams is currently considered theonly available remedy to the problem of mercury-induced corrosion duringthe liquefaction of natural gas.

Whilst a Joule-Thompson valve 20 is used in the first preferredembodiment of the present invention, any suitable expansion valve may beemployed; for example, a turbo-expander or other means of isotropicallyexpanding the gas into the cooling vessel 11 to effect cooling of thenatural gas feed stream 12 into the cooling vessel 11. The process ofexpansion cools the natural gas feed stream 12 entering the coolingvessel 11 at the inlet 20 to between −100° C. and −125° C. The inletpressure at the feed pipe 22 of between 200 to 600 psia is reduced tobetween 150 and 250 psia within the cooling vessel 11.

A second preferred embodiment of the present invention is depicted inFIG. 2. In this embodiment, a stream of sub-cooled LNG 26 is introducedto the cooling vessel 110 via a second inlet 28. The sub-cooled LNGstream 26 is introduced in part to assist in cooling the expandednatural gas feed stream 12 which is entered the cooling vessel 11through the expansion valve 20 down to a temperature of at least −140°C. The natural gas feed stream 12 comprising the freezable speciespartially liquefies upon contact with the sub-cooled LNG stream 26introduced by the second inlet 28. As LNG begins to form, by definition,solids of the freezable species also form creating a volume ofpressurised LNG 14 within the cooling vessel 11.

The sub-cooled LNG stream 26 may be recycled following separation ofsolids of the freezable species from the slurry removed from the coolingvessel 10 or be recycled from the product stream 30. Depending on thedegree of sub-cooling required, the recirculating load of LNG to therecycle stream 22 may be many multiples of the amount required forcustomer use. A second recycle strewn 23 as depicted in FIG. 4 may beinjected into the cooling vessel through an inlet 25 adapted to betangential to and located near a top portion of the cooling vessel 10 tocreate the vortex 30 within the cooling vessel 10.

The second inlet 28 may be adapted to be tangential to the coolingvessel 11. With a tangential second inlet 28 a vortex 30 is createdwithin the volume of pressurised LNG 14 in the cooling vessel 11. It hasbeen found that best results for creating the vortex is achieved whenthe second inlet 28 is located at or near the uppermost level 29 of thevolume of the pressurised LNG 14.

Once solids of the freezable species form within the volume of thepressurised LNG 14, the volume of pressurised LNG 14 is referred to as aslurry.

The solids of the freezable species are more dense than the pressurisedLNG. The solid state density of CO₂ is about 1.2 g/cm³ compared with thedensity of LNG which is 0.44 g/cm³. Thus the solid state density of CO₂,for example, is four times higher than that of LNG. The solids thusmigrate under gravity towards the lowermost portion 31 of the coolingvessel 11 in the direction of the outlet 32.

The creation of a vortex 30 is understood to assist in accumulatingsolids of the freezable species towards the centre of the cooling vessel11 and also to encourage the migration of the solids of the freezablespecies under gravity towards the outlet 32 at the bottom of the coolingvessel 11. It is understood that the creating of a vortex 30 encouragesseparation I the same way as the method of density separation occurringwithin a hydrocyclone.

The slurry thus becomes more concentrated in solids towards the outlet32 than towards the uppermost level 29 of the volume of pressurised LNG14 within the cooling vessel 11. Thus the highest purity of pressurisedLNG produced within the cooling vessel 11 will be towards the uppermostlevel 29 of the volume of pressurised LNG 14. A product stream ofpressurised LNG 33 is removed at or near the uppermost level 29 of thevolume of pressurised LNG 14. The product stream 33 may be furthercooled to a temperature and pressure suitable for the desired method ofsport and may also be subject to additional solid separation stages (notshown) if required. Multiple cascaded separators may be required toprovide the degree of separation needed. Such traditional separators maybe provided either in series or in parallel.

It has been described above that one method of creating the vortex 30within the volume of pressurised LNG 14 is to introduce the subcooledLNG stream 26 to the cooling vessel via a tangential second inlet 28.Another method of creating a vortex is to provide a stirrer or othersuitable mechanical agitation means 34 preferably towards the lowermostlevel 31 of the cooling vessel 11 as depicted in FIG. 2.

The slurry 24 is removed from the cooling vessel 11 via outlet 32.Typically, the slurry 24 will be in the temperature range of −130° C.and −150° C. at a pressure of between 150 to 250 psia. Tests conductedby the applicant have indicated that for a natural gas feed streamcontaining 21% CO₂ at a temperature of −52° C. that has been cooled inan arrangement as depicted in FIG. 2, including the introduction of asub-cooled LNG stream at a temperature of −160° C., resulted in themajority of the CO₂ forming solids which are removed at the outlet 32.In the tests conducted by the applicant, the concentration of thepressurised LNG product stream 30 was reduced down to a level of 0.2%CO₂.

In the embodiment depicted in FIG. 2, the solids of the freezablespecies as separated from the slurry 24 using a cyclone 16. Whilst acyclone is the preferred means for effecting solid-liquid separation,any suitable means for solid-liquid separation may be used such as agravity separator or a combination of gravity and hydrocyclone methods.One or more cyclones 16 in series or parallel may also be employeddownstream of the cooling vessel. In a third preferred embodiment of thepresent invention as depicted in FIG. 3, the slurry 24 is passed througha cyclone 16 which is an integral part of the cooling vessel 11 andforms the outlet 32.

In a fourth preferred embodiment of the present invention as depicted inFIG. 4, the cooling vessel 11 includes an integral hydrocyclone 16through which the slurry 34 exits the cooling vessel 11. The slurry 34is then transferred to a solids collection vessel 42 in fluidcommunication with the cooling vessel 110 via transfer means 36 in theform of a screw conveyor. Any suitable means for transferring of theslurry from the cooling vessel 11 to the solids collection vessel 42 maybe employed, such as an incline screw conveyor 36, an auger or standardconveyor.

In accordance with the fourth preferred embodiment of the presentinvention, the slurry 34 is continuously removed from the cooling vessel11 through the integral hydrocyclone 16. The solids collection vessel 42and cooling vessel 10, as well as screw conveyor 36, are maintained atthe same working pressure. In this way, continuous removal of the solidsof the freezable species from the cooling vessel 11 may be effected.

The screw conveyor 36 may be driven either externally or internally byuse of direct drive shaft 38. If the drive shaft 38 is locatedinternally of the transfer means 36, the motor and gearbox for the driveshaft would be subjected to a continuous exposure to cryogenic pressuresand temperatures and pressurised LNG which would preclude the use ofrotating seals. It is understood that the reliability of rotatingequipment at cryogenic temperatures is generally poor. In order toovercome this problem, an extended drive shaft may be employed such thatthe motor is external to the transfer means and not exposed to cryogenictemperatures and the working pressure of all three units. In any event,all seals used to drive the screw conveyor 36 must be able to withstandthe working pressure of the transfer means, cooling vessel and solidscollection unit.

The screw conveyor 36 is mounted at an angle to assist in the drainingof LNG from the slurry. Typically the angle of inclination of the screwconveyor 36 is in the order of 60°. However, it is to be clearlyunderstood that the exact angle of inclination of the screw conveyor 36is not critical to the present invention. As the outlet slurry 32 iscarried by the screw conveyor 36 to a level 37 higher than the liquidlevel 30 of the cooling vessel 10, a capillary action results inseparation of the LNG from the slurry resulting in an increase in theconcentration of solids within the slurry forming a slush 40.

The slush 40 is thus more concentrated in solids than the slurry 34 thatleaves the cooling vessel 11. The slush 40 enters the solids collectionvessel 42 and is then heated to convert the solids of the freezablespecies to a liquid form within the solids collection vessel 42. Onesuitable solids collection vessel would be a reboiler. Alternatively,the slush 40 collected in the solids collection vessel 42 may be heatedby means of introducing a process stream at a higher temperature thanthat of the slurry stream entering the solids collection vessel 42.

A rotating roller (not shown) located at the exit of the hydrocyclone 16may be used to create a seal between the cooling vessel 11 and thesolids collection vessel 42. The solids collection vessel 42 wouldtypically be clearance-fit with respect to its casing 44 to allow spacefor the LNG to drain. Best results are obtained when the screw conveyor46 is arranged to be off-centre to provide the least clearance on thesolids side while allowing plenty of space for the LNG to drain on theother side. A bush or bearing or other suitable rotation control meansis provided at the top and bottom ends of the screw conveyor 36 tocontrol its rotation and end thrust. For best results, the bottombearing is such that the screw conveyor 36 is sealed at the bottom.

Once the solids collected in the solids collection vessel 42 areconverted to liquid form, the liquefied freezable species is dischargedthrough an outlet 46 of the solids collection vessel 42. The removal ofthe liquefied freezable species from the screw conveyor 42 via outlet 46may be either conducted on a continuous basis or as a batch operation,depending on the level of the slush 40 in the reboiler 42. The outletstream of the liquefied freezable species may be used for heat recoveryor injected back into a disposal well. In particular, liquefied CO₂ maybe used to advantage for other heat exchangers required in othersections of the LNG plant. Alternatively, the liquefied CO₂ may be usedfor a seabed heat exchanger as a cost-effective alternative torecompression equipment.

During the step of heating to convert the solids of the freezablespecies to liquid form, the LNG remaining in the slush 40 is driven offas a natural gas vapour stream 50. The natural gas vapour stream 50 maythen be returned to the cooling vessel 11 via inlet 52. To minimise thequantity of vapour fed to the cooling vessel natural gas through inlet52, it is important that the maximum possible amount of LNG is allowedto drain from the slurry 34 entering the screw conveyor 36 before theslush 40 enters the solids collection vessel 42.

The reboiler 42 may be heated using electrical heating controlled viathermostat. The nominal working pressure of the cooling vessel being 200psia, the thermostat would be set at −30° C. in order to convert solidsof CO₂ to liquid. The heating system used to heat the reboiler 42 shouldbe designed so as to gently warm the slush 40 to avoid hot spots formingwithin the slush. A stirrer (not shown) may be provided within thereboiler 42 to avoid such hot spots forming within the heated slush.

In order to facilitate continuous removal of the freezable species, theworking pressure of the solids collection vessel 42, transfer means 36and cooling vessel 11 must be maintained at the same working pressure.It is highly desirable that the working pressure be maintained above thetriple-point pressure of the freezable species. In the case of CO₂, thetriple-point pressure at the temperatures of liquefaction will be in theorder of 75 psia. In normal operation the cooling vessel 11 along withthe solids collection vessel 42 and transfer means 36 should be operatedat a pressure of around 200 psia. If the solids of the freezable speciesare allowed to melt at pressures below the triple-point pressure of thefreezable species, an undesirable vapour phase of the freezable specieswould be produced.

The cooling vessel 11 is constructed in such a way that at least thatpart of the internal wall of the cooling vessel 11 surroundingsolidification zone 13 is constructed from a material having a low heattransfer coefficient. Such a choice of materials of construction for thecooling vessel is a radical departure from convention material selectionpractice for liquefaction of natural gas. Selecting the material ofconstruction having a low heat transfer coefficient for at least thatpart of the cooling vessel that surrounds the solidification zone 13results in a thermal gradient within the cooling vessel 11 whereby thetemperature towards the centre of the cooling vessel 11 is at all timescooler than the temperature at the walls of the cooling vessel 11.

In accordance with classical nucleation theory, solids formpreferentially under conditions that result in the greatest possiblereduction in the overall energy of the system. In the absence of specialconditions, solidification would typically occur at the walls of thecooling vessel as solidification of the surface requires less surfacearea per unit volume to form a solid particle than does the nucleationof a solid away from a surface. Without wishing to be bound by theory,it is understood that several mechanisms within the cooling vessel arecontributing to the formation of solids away from the walls of thecooling vessel.

The prototype cooling vessel constructed by the applicant for testing ofthe present invention was constructed of highly polished syntheticsingle crystal sapphire. Sapphire was chosen in order to provide a meansfor observing the solidification of the freezable species within thecooling vessel. A surprising outcome of the observations was that thechoice of single crystal sapphire as the material of constructionresulted in solids forming away from the walls of the cooling vessel 11.It is to be clearly understood, however, that the present invention isnot limited in its scope to the selection of sapphire as the material ofconstruction of the cooling vessel. Any other suitable material having alow heat transfer coefficient is sufficient Such a material may be ametal oxide or a ceramic such as partially stabilised zirconia.

The particular material of construction used during testing had a highlevel of anisotropy. It is understood that this property of ananisotropic growth habit of the single crystal is understood to havebeen one of the other factors that contributed to discouraging solidsformation occurring at the walls of the cooling vessel. Additionally,the single crystal sapphire was highly polished and a polished sapphiresurface is ranked as one of the smoothest known amongst any material. Itis considered that polishing of at least the internal surface of thematerial of construction of the wall in the solidification zone is oneof the factors that contributes to solids forming preferentially awayfrom the walls of the cooling vessel.

It is understood that yet another factor encouraging solid formation tooccur away from the walls of the cooling vessel is the differentialsurface tension that arises due to the thermal gradient that is inducedwithin the cryogenic liquid. A liquid at a lower temperature is known tohave a higher surface tension than a liquid at a higher temperature. Byconstructing at least part of the wall surrounding the solidificationzone from a material having a low heat transfer coefficient, thetemperature of the LNG is cooler towards the centre of the coolingvessel and thus the surface tension of the liquid towards the centre ofthe cooling vessel is higher. Again, to encourage an overall reductionin the energy of the system, the formation of solids towards the centreof the cooling vessel is encouraged.

It is worth noting that solids were observed to form on the walls of thecooling vessel when a vortex was not created within the pressurised LNG.However, the solids were a very low percentage of the overall solidsformed within the cooling vessel and exhibited a planar growth habit Thesolids forming on the walls were readily detached from the walls withdetachment being observed to occur due to thermocapillary motion of thefluid itself within the cooling vessel, even if a vortex was notcreated. When a vortex was created within the volume of pressurised LNGwithin the cooling vessel, solids of the freezable species were notobserved at any time to form on the walls of the cooling vessel.

Examples of the test work conducted using the Sapphire Cell will now bedescribed in order to provide a better understanding of the presentinvention. These examples are not to be taken as limiting the inventionin any way and are provided for illustrative purposes only.

EXAMPLES

Tests were conducted on a feed gas containing 25% CO₂ introduced at 280psia and −140° C. Using the method described above, the CO₂ content wasreduced from 25% to 0.29%. The feed gas contained the following:Component Mole Fraction N₂ 1.939 CO₂ 24.95 C1 64.64 C2 5.493 C3 2.385IC₄ 0.239 NC₄ 0.292 IC₅ 0.038 NC₅₊ 0.023Note:The gas includes parts per million amounts of mercaptans.

After testing, the GC analysis of the LNG produced following separationof the solid contaminants at 145 psia and −140° C. reads as follows:Component Mole Fraction N₂ 1.28 CO₂ 0.29 C1 94.65 C2 4.48 C3 2.02 IC₄0.21 NC₄ 0.27 IC₅ 0.04 NC₅₊ 0.03

The mole fraction of CO₂ has been reduced substantially from 24.95% inthe feed stream to only 0.29% in the LNG outlet stream. The solidscollected had the following composition: Component Mole Percentage CO₂95.37 C1 0.37 C2 0.06 C3 0.66 IC₄ 0.90 NC₄ 1.92 IC₅ 0.36 NC₅ 0.24 C₆0.11

It will be readily apparent to a person skilled in the relevant art thatthe present invention has significant advantages over the prior artincluding, but not limited to, the following:

-   -   a) a low cost liquefaction and refrigeration process which        significantly enhances the economics of small scale PLNG        production;    -   (b) small-scale LNG plants based on the process of the present        invention become competitive with large-scale projects on a        specific capital cost basis ($/tpy) and on a total production        cost basis ($/GJ);    -   (c) A wide variation in feed gas compositions can be processed;        and    -   (d) the process is simpler to operate and maintain than the        conventional pre-treatment process.

Now that an embodiment of the present invention has been described indetail, it will be apparent to those skilled in the relevant arts thatnumerous modifications and variations may be made without departing fromthe basic inventive concepts. In particular, whilst accommodation of ahydrocyclone fitted to the bottom of the cooling vessel in combinationwith an inclined auger and reboiler have been described in the preferredembodiment of the present invention, other means for removing the solidsfrom the bottom of the cooling vessel and separating the solids may beused and equally fall within the scope of the present invention. Forexample, a rotating high gravity separator in the form of a centrifugemay be provided for continuous separation of the liquid/solid mixture.The solid/liquid separation may then be achieved using filtration; forexample, by means of a particle trap provided with the rotary scraper.Also, while the technology is particularly intended for use forsmall-scale LNG production facilities, it is equally applicable tolarge-scale and offshore LNG production. All such variations andmodifications are to be considered within the scope of the presentinvention, the nature of which is to be determined from the foregoingdescription.

1. A method for removal of a freezable species from the natural gas feedstream, the method comprising the steps of: cooling the feed stream in acooling vessel to produce pressurised LNG in a manner such that thefreezable species solidify forming a slurry with the pressurised LNG;and, removing the slurry from the cooling vessel whilst maintaining thefreezable species as a solid.
 2. A method for removal of a freezablespecies as defined in claim 1 wherein the step of cooling is conductedso as to maintain a temperature gradient within the cooling vessel suchthat the temperature towards the centre of the cooling vessel is lessthan the temperature at the wall of the cooling vessel.
 3. A method forremoval of a freezable species as defined in claim 1 further comprisingthe step of separating the solids of the freezable species from theslurry.
 4. A method for removal of a freezable species as defined inclaim 3 wherein the step of removing the slurry from the cooling vesselis conducted simultaneously with the step of separating the freezablespecies from the slurry.
 5. A method for removal of a freezable speciesaccording to claim 1 further comprising the step of recycling to thecooling vessel LNG from which the freezable species has been separated.6. A method for removal of a freezable species according to claim 1further comprising the step of liquefying the separated solid of thefreezable species.
 7. A method for removal of a freezable species asdefined in claim 6 wherein further comprising the step of recycling tothe cooling vessel natural gas from which the freezable species has beenseparated during the step of liquefying.
 8. A method for removal of afreezable species according to claim 1 further comprising the step ofcreating a vortex within the cooling vessel.
 9. A method for removal ofa freezable species as defined in claim 8 wherein the vortex is createdby stirring the slurry.
 10. A method for removal of a freezable speciesaccording to claim 8 wherein the vortex is created by one or both of (a)stirring the slurry; and, (b) introducing a fluid stream tangentially tothe cooling vessel.
 11. A method for removal of a freezable species asdefined in claim 10 wherein the fluid stream introduced tangentially tothe cooling vessel is a stream of sub-cooled LNG.
 12. A method forremoval of a freezable species as defined in claim 11 wherein the streamof sub-cooled LNG may be the sub-cooled LNG stream recycled afterseparation of the freezable species from the slurry.
 13. A method forremoval of a freezable species according to claim 1 wherein the step ofcooling comprises the step of isotropically expanding the feed stream.14. A method for removal of a freezable species according to claim 5wherein the step of cooling comprises one or both of (a) isotropicallyexpanding the feed stream; and, (b) introducing a stream of sub-cooledLNG.
 15. A method for removal of a freezable species as defined in claim14 wherein the stream of sub-cooled LNG is the stream of recycled LNGseparated from the slurry during the step of separating the solids ofthe freezable species.
 16. A method for the continuous removal of afreezable species from a natural gas feed stream comprising the stepsof: cooling the feed stream in a cooling vessel to produce pressurisedLNG in a manner such that the freezable species solidify forming aslurry with the pressurised LNG; and, separating the solids of thefreezable species from the slurry, wherein the step of cooling and thestep of separating are conducted at the same working pressure.
 17. Amethod for the continuous removal of a freezable species as defined inclaim 16 wherein the steps of cooling and separating are conducted atthe same pressure in use.
 18. A method for the continuous removal of afreezable species as defined in claim 16 further comprising the step ofheating the separated solids of the freezable species to form a liquidof the freezable species.
 19. A method for the continuous removal of afreezable species as defined in claim 18 wherein the steps of cooling,separating and heating are conducted at the same pressure in use.
 20. Amethod for continuous removal of a freezable species as defined in claim17 wherein the pressure is maintained at all times below thetriple-point pressure of the freezable species.
 21. A method for thecontinuous removal of a freezable species as defined in claim 16 whereinthe step of cooling is conducted so as to maintain a temperaturegradient within the cooling vessel such that the temperature towards acentre of the cooling vessel is less than the temperature at a wall ofthe cooling vessel.
 22. A method for continuous removal of a freezablespecies as defined in claim 16 further comprising the step of removingthe slurry from the cooling vessel.
 23. A method for continuous removalof a freezable species as defined in claim 22 wherein the step ofremoving the slurry from the cooling vessel is conducted simultaneouslywith the step of separating the freezable species from the slurry.
 24. Amethod for continuous removal of a freezable species as defined in claim16 further comprising the step of recycling to the cooling vessel LNGfrom which the freezable species has been separated.
 25. A method forcontinuous removal of freezable species as defined in claim 16 furthercomprising the step of liquefying the separated solid of the freezablespecies.
 26. A method for continuous removal of a freezable species asdefined in claim 25 wherein further comprising the step of recycling tothe cooling vessel natural gas from which the freezable species has beenseparated during the step of liquefying.
 27. A method for continuousremoval of a freezable species as defined in claim 16 further comprisingthe step of creating a vortex within the cooling vessel.
 28. A methodfor continuous removal of a freezable species as defined in claim 27wherein the vortex is created by stirring the slurry.
 29. A method forcontinuous removal of a freezable species as defined in claim 27 whereinthe vortex is created by one or both of (a) stirring the slurry; and,(b) introducing a fluid stream tangentially to the cooling vessel.
 30. Amethod for continuous removal of a freezable species as defined in claim29 wherein the fluid stream introduced tangentially to the coolingvessel is a stream of sub-cooled LNG.
 31. A method for continuousremoval of a freezable species as defined in claim 30 wherein the streamof sub-cooled LNG may be the sub-cooled LNG stream recycled afterseparation of the freezable species from the slurry.
 32. A method forcontinuous removal of a freezable species as defined in claim 16 whereinthe step of cooling comprises the step of isotropically expanding thefeed stream.
 33. A method for continuous removal of a freezable speciesas defined in claim 16 wherein the step of cooling comprises one or bothof (a) isotropically expanding the feed stream; and, (b) introducing astream of sub-cooled LNG.
 34. A method for continuous removal of afreezable species as defined in claim 33 wherein the stream ofsub-cooled LNG is the stream of recycled LNG separated from the slurryduring the step of separating the solids of the freezable species. 35.An apparatus for removing a freezable species from a natural gas feedstream, the apparatus comprising: a cooling vessel having asolidification zone therewithin wherein a part of the cooling vesselthat surrounds the solidification zone is constructed from a materialhaving a low thermal conductivity; an inlet for introducing the feedstream to the cooling vessel; and, an outlet for removing a slurry ofsolidified freezable species and pressurised LNG from the coolingvessel.
 36. An apparatus for removing a freezable species as defined inclaim 35 further comprising a solid/liquid separator for separating thesolidified freezable species from the slurry.
 37. An apparatus forremoving a freezable species as defined in claim 36 wherein theseparator is located at and/or defines the outlet.
 38. An apparatus forremoving a freezable species as defined in claim 36 wherein theseparator may be one of a plurality of separators arranged in series orin parallel.
 39. An apparatus for removing a freezable species asdefined in claim 35 further comprising an expansion valve located atand/or defining the inlet for introducing the feed stream to the coolingvessel.
 40. An apparatus for removing a freezable species as defined inclaim 39 wherein the expansion valve is a Joule-Thompson valve.
 41. Anapparatus for removing a freezable species as defined in claim 35further comprising a stirrer for creating a vortex within the coolingvessel in use.
 42. An apparatus for removing a freezable species asdefined in claim 35 wherein the inlet is configured to introduce thefeed stream tangentially to an internal wall of said cooling vessel. 43.An apparatus for removing a freezable species as defined in claim 35wherein the material of construction of an internal wall of the coolingvessel is polished.
 44. An apparatus for removing a freezable species asdefined in claim 43 wherein the internal wall is highly polished.
 45. Anapparatus for removing a freezable species as defined in claim 35wherein the material of construction of an internal wall of the coolingvessel is anisotropic.
 46. An apparatus for removing a freezable speciesas defined in claim 35 wherein the material of construction on aninternal wall of the cooling vessel is a metal oxide.
 47. An apparatusfor removing a freezable species as defined in claim 35 wherein thematerial of construction of an internal wall of the cooling vessel is aceramic.
 48. An apparatus for removing a freezable species as defined inclaim 35 wherein the material of construction of an internal wall of thecooling vessel is a single crystal.
 49. An apparatus for removing afreezable species as defined in claim 35 wherein the material ofconstruction of an internal wall of the cooling vessel is sapphire. 50.An apparatus for continuously removing a freezable species from anatural gas feed stream, the apparatus comprising: a cooling vesselhaving a solidification zone therewithin wherein a part of the coolingvessel that surrounds the solidification zone is constructed from amaterial having a low thermal conductivity; an inlet for introducing thefeed stream to the cooling vessel; an outlet for removing a slurry ofsolidified freezable species and pressurised LNG from the coolingvessel; and a solids collection vessel in fluid communication with thecooling vessel.
 51. An apparatus for continuously removing a freezablespecies as defined in claim 50 further comprising a transfer means fortransferring the slurry from the cooling vessel to the solids collectionvessel.
 52. An apparatus for continuously removing a freezable speciesas defined in claim 51 wherein the transfer means is inclined at anangle.
 53. An apparatus for continuously removing a freezable species asdefined in claim 52 wherein the angle is not less than 60° to thehorizontal reference plane.
 54. An apparatus for continuously removing afreezable species as defined in claim 52 wherein the transfer means isprovided with an external drive.